The emergence of hybrid halide perovskite compounds, AMX3 (A=Cs+, CH3NH3+, or HC(NH2)2+; M=Sn2+ and Pb2+; and X=Cl−, Br−, and I−), in solid-state solar cells has triggered a phenomenal advance in photovoltaic efficiency. Perovskite compounds, in the form of CH3NH3PbX3, have been employed as light-absorbing materials in liquid dye-sensitized solar cells (Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc. 2009, 131, 6050). However, they did not engender significant attention because of short device lifetime resulting from the fast dissolution of perovskites in the redox electrolyte solution. Later, perovskite based solar cells have returned in a different fashion: solid-state. The photovoltaic performance race has been remarkable ever since, and an efficiency of 20.1% has been certified by the National Renewable Energy Laboratory (NREL) (National Renewable Energy Laboratory, N.R.E.L.; Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Science 2014, 345, 542). Among the light absorber candidates, 3D methylammonium (MA) lead iodide (MAPbI3) is a prominent choice owing to its outstanding properties as a solar cell absorber, including a high extinction coefficient, a medium band gap, a small exciton binding energy, and long exciton and charge diffusion lengths. From a commercialization point of view, the large-scale implementation of perovskite solar cells requires toxicity and stability issues to be resolved. Works on tin-based perovskites have been reported, demonstrating a promising efficiency of ca. 5% for the CH3NH3SnI3-xBrx system (Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Nat. Photonics 2014, 8, 489; Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A. A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; Petrozza, A.; Herz, L. M.; Snaith, H. J. Energy Environ. Sci. 2014, 7, 3061), as well as in the mixed-metal CH3NH3Sn1-xPbxI3 system (Ogomi, Y.; Morita, A.; Tsukamoto, S.; Saitho, T.; Fujikawa, N.; Shen, Q.; Toyoda, T.; Yoshino, K.; Pandey, S. S.; Ma, T.; Hayase, S. J. Phys. Chem. Lett. 2014, 5, 1004; Hao, F.; Stoumpos, C. C.; Chang, R. P.; Kanatzidis, M. G. J. Am. Chem. Soc. 2014, 136, 8094; Zuo, F.; Williams, S. T.; Liang, P.-W.; Chueh, C.-C.; Liao, C.-Y.; Jen, A. K. Y. Adv. Mater. 2014, 26, 6454). The moisture instability of MAPbI3, however, has been poorly addressed. Recently, Smith et al. reported the solar cell application of a layered (PhC2H5NH3)2(CH3NH3)2Pb3I10 perovskite light absorber with enhanced moisture stability (Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. Angew. Chem., Int. Ed. 2014, 53, 11232).
From a fundamental point of view, efficient external luminescence is an indirect indication of accessing the highest possible open-circuit voltage, a major factor in the total power output aside from the short-circuit current and fill factor. The 2D A2MI4-based perovskite compounds, where M is a divalent group 14 (Mitzi, D. B. Chem. Mater. 1996, 8, 791) or lanthanide (Mitzi, D. B.; Liang, K. Chem. Mater. 1997, 9, 2990) metal, have been reported to display high photoluminescence (PL) at room temperature, and until now, they have been employed in field-effect transistor (FET) (Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. Science 1999, 286, 945) and light-emitting diode (LED) devices (Ishihara, T.; Takahashi, J.; Goto, T. Solid State Commun. 1989, 69, 933; Wu, X. X.; Trinh, M. T.; Niesner, D.; Zhu, H. M.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X. Y. J. Am. Chem. Soc. 2015, 137, 2089).
There are mainly two varieties of Ruddlesden-Popper (RP) perovskites that have been employed in devices: phenylethylammonium-(PEA), derived by the original work of Calabrese et al. and butylammonium-based (BA) derived from the pioneering work of Ishihara and co-workers on aliphatic ammonium cations. (Calabrese, J.; Jones, N. L.; Harlow, R. L.; Herron, N.; Thorn, D. L.; Wang, Y., Preparation and Characterization of Layered Lead Halide Compounds. Journal of the American Chemical Society 1991, 113 (6), 2328-2330; and Ishihara, T.; Takahashi, J.; Goto, T., Exciton-State in Two-Dimensional Perovskite Semiconductor (C10H21NH3)2PbI4. Solid State Communications 1989, 69 (9), 933-936.) In both series of compounds, it has been shown that by increasing the number of perovskite layers (n-number in the chemical formula), the photovoltaic properties improve with the BA series. In the majority of the previously reported studies, however, the absorption spectra contain clear traces of several excitonic features characteristic of the coexistence of various n-members, a fact that confirms the forecast that “Compounds with n>3 cannot be isolated in a pure form . . . ”. (Papavassiliou, G. C., Three- and Low-Dimensional Inorganic Semiconductors. Prog Solid State Ch 1997, 25 (3-4), 125-270.) This is mainly because the difference in the thermodynamic stability of the higher n-members is small and it becomes even smaller as one approaches n=∞ making difficult to isolate them in a pure form. A similar effect is well-known for the oxide RP perovskites where it has been shown that higher n-members tend to disproportionate to the n=3 and n=∞ members which act as thermodynamic sinks.(Yan, L.; Niu, H. J.; Duong, G. V.; Suchomel, M. R.; Bacsa, J.; Chalker, P. R.; Hadermann, J.; van Tendeloo, G.; Rosseinsky, M. J., Cation ordering within the perovskite block of a six-layer Ruddlesden-Popper oxide from layer-by-layer growth—artificial interfaces in complex unit cells. Chemical Science 2011, 2 (2), 261-272.)